[opt](FileReader) InMemoryReader is only used in s3 (#23486)
If file size < 8MB, the file will be read into memory, and this idea is from https://github.com/apache/hadoop/blob/trunk/hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/prefetching.md#s3inmemoryinputstream. However, in some cases, we only read one or two columns in a file, and the actually required bytes is only 1%, resulting in a multiple fold increase in the amount of data read. Therefore, `InMemoryReader` can only used in object storage, and reduce the threshold.
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